This invention relates to a potential measurement device using an electron beam and in particular to a stroboscopic type potential measurement device permitting the measurement of variations of potentials (potential waveforms) on interconnection lines in LSI by using charged particles of a scanning electron microscope and so forth.
Initially, the principle of the stroboscopic type potential measurement device will be explained.
FIG. 1 is a schematic illustrating the fundamental construction of a stroboscopic type scanning electron microscope for measuring potential waveforms by using a pulsed electron beam. An electron beam 2 emitted by an electron gun 1 is focused on a sample under test 10 by means of an electron lens 6 and scanned by means of a deflector or scanning coil 8 a manner similar to a television display tube. When an electron beam collides against a solid body, reflected electrons or secondary electrons are produced. These are detected by a detector 9 and their image is displayed on a display device 7. This is the principle of the scanning electron microscope.
When a sample varying with a high speed is observed by means of this scanning electron microscope, the scanning of the electron beam by the deflector 8 cannot follow the variations of the sample therefore, all the variations are displayed superposed on each other. Consequently a pulse gate (combination of the deflector 3 and an aperture 4) is added, which chops the electron beam by means of a pulse generator synchronized with a driving device 11 giving the variations to the sample. By using such a construction the electron beam, with which the sample is scanned, can be controlled so that the sample is irradiated therewith only at a predetermined phase of the variations of the sample in order to detect the state of the sample only at the moment that the sample is irradiated. FIGS. 2A and 2B are schemes for explaining the above mentioned operation. The abscissa of FIG. 2A represents variations of the state of the sample, where the object is displaced between A and C, and the ordinate the time. Supposing that the phase (Timing), where the sample is irradiated with the electron beam, is a point of time a, the sample is observed on the display device 7, as indicated in FIG. 2B (a). Similarly, when the phase of irradiation is points of time b and c in FIG. 2A, the sample is observed, as indicated in FIG. 2B (b) and (c), respectively. The phase, where the sample is observed, can be selected by means of a phase shifter 5 indicated in FIG. 1. In general, the phase shifter 5 is constituted by combinations of delay lines.
Principal applications of the stroboscopic type scanning electron microscope are observations of potential varying with a high speed in LSI. In this case an energy analyzer for secondary electrons is added between the detector 9 and the sample 10.
FIG. 3A illustrate this principle, in which a control electrode 13 is disposed between the sample under test 10 and the detector 9 opposed thereto. The control electrode 13 forms a potential barrier for discriminating energy of secondary electrons emitted by the sample 10 irradiated with the electron beam 2. FIG. 3B is a scheme for explaining the operation of this potential barrier. In the case where no control electrode 13 is disposed over the sample, all the secondary electrons are detected by the detector 9. Energy of the secondary electrons emitted by the sample 10 of zero potential is distributed as indicated by A in FIG. 3B. When the potential of the sample 10 is -5 V, the distribution is changed as indicated by B. When the control electrode 13, to which -5 V is applied, is disposed, since detected secondary electrons are limited to those whose energy is higher than 5 eV, the amount of the detected secondary electrons varies, depending on the potential of the sample 10. Since the amount of the detected secondary electrons depends on the sample potential, inversely the potential of the sample 10 can be known from the amount of the detected secondary electrons.
However, by the potential measurement, for which only the control electrode 13 described above is dispcsed, the relationship between the potential of the sample 10 and the amount of the detected secondary electrons is not linear and therefore it is difficult to measure the potential quantitatively.
Consequently, in order to obtain a linear relationship therebetween, a feedback method is utilized, by which the potential of the control electrode is so regulated by means of an electronic circuit that the amount of the detected secondary electrons is maintained at a constant. (cf. IEEE, Journal of Solid State Circuits, vol. SC-13, No. 3, 1978).
FIG. 4 is a block diagram for explaining this feedback method. The output of the detector 9 is compared with a voltage of a reference voltage source 15 and the difference therebetween is amplified by an amplifier 14, the output of which is given to the control electrode 13. Since the feedback circuit is so constructed that, when the amount of the detected secondary electrons increases, the potential of the control electrode 13 decreases so that increase of the amount is suppressed, the amount of the detected secondary electrons maintained constant, however the potential of the sample 10 varies. In this way, since variations in the potential of the sample correspond one-to-one to variations in the potential of the control electrode 13, variations in the unknown potential of the sample can be determined quantitatively by measuring the potential of the control electrode 13.
When the phase of the phase shifter 5 indicated in FIG. 1 is varied slowly in a range from 0.degree. to 360.degree., while measuring quantitatively the potential in the manner described above, a potential waveform corresponding to phase variations can be obtained. Regulation of the phase is effected by using a delay line, as stated previously. In this way, by using a stroboscopic type scanning electron microscope, it is possible to measure potential waveforms as by a sampling oscilloscope.
By the method described above it is possible to measure potential waveforms by means of an electron beam. It has been already stated that principal applications of the stroboscopic type scanning electron microscope are measurements of potential waveforms in LSIs. However, in many cases, an LSI is covered with an insulation film called passivation for protecting semiconductor circuits formed therein against dirt and humidity. Since this passivation film lies between the electron beam and the metallic electrodes in the LSI, it acts as a capacitor. Due to this interposition of a capacitance it is not possible to measure any stationary voltages (e.g. DC voltage). Consequently it is not possible to obtain any potential waveform by the method described above, in which the phase is regulated slowly. This aspect will be explained below, referring to FIGS. 5, 6A, 6B and 6C. FIG. 5 indicates a model representing the relation between the action of the capacitor formed by the passivation film and obtained signals. An electric current I 17 flows through a resistance 16 from an AC voltage source 21 through an electrostatic capacitance 20 formed between a spot irradiated with the electron beam and a conductor 19 in the LSI. The resistance 16 is an equivalent detection resistance. The amplitude of the AC voltage source 21 in this model is equal to the voltage in the LSI. However its frequency is not equal to the frequency in the LSI, instead the phase of the former is shifted by 360.degree. with respect to that of the latter. In the case where the phase is fixed, a DC voltage is obtained. As it can be seen from this equivalent circuit, when the period for changing the phase is made as short as possible, the original waveform (waveform to be measured in the LSI) can be measured. Therefore, the inventors of this invention have ameliorated this method usually utilized, by which the whole phase is scanned in 10-40 seconds, and realized a method, by which the whole phase is scanned in 10-40 ms and measured values thus obtained are superposed about 1000 times (cf. Scanning Electron Microscopy, 1983, Vol. II, pp. 561-568. However, even by this method, when the passivation film is thick and conductors in the LSI still exist fine, difficulties are encasticed. An attempt to make the period for changing the phase still shorter causes the division of the phase, i.e. the division in time become coarse and thus is becomes impossible to measure waveforms in detail.